Due to extremely poor electrical conductivity of all cathode (positive electrode) active materials in a lithium-ion, lithium metal, or lithium-sulfur cell, a conductive additive (e.g. carbon black, fine graphite particles, expanded graphite particles, or their combinations), typically in the amount of 5%-20%, must be added into the electrode. In the case of a lithium-sulfur cell, a carbon amount as high as 50% by weight is used as a conductive support for sulfur in the cathode. However, the conductive additive is not an electrode active material (i.e. it is not capable of reversibly storing lithium ions). The use of a non-active material means that the relative proportion of an electrode active material, such as LiFePO4, is reduced or diluted. For instance, the incorporation of 5% by weight of PVDF as a binder and 5% of carbon black as a conductive additive in a cathode would mean that the maximum amount of the cathode active material (e.g., lithium cobalt oxide) is only 90%, effectively reducing the total lithium ion storage capacity. Since the specific capacities of the more commonly used cathode active materials are already very low (140-170 mAh/g), this problem is further aggravated if a significant amount of non-active materials is used to dilute the concentration of the active material.
State-of-the-art carbon black (CB) materials, as a conductive additive, have several drawbacks:    (1) CBs are typically available in the form of aggregates of multiple primary particles that are typically spherical in shape. Due to this geometric feature (largest dimension-to-smallest dimension ratio or aspect ratio ˜1) and the notion that CBs are a minority phase dispersed as discrete particles in an electrically insulating matrix (e.g. lithium cobalt oxide and lithium iron phosphate), a large amount of CBs is required to reach a percolation threshold where the CB particles are combined to form a 3-D network of electron-conducting paths.    (2) CBs themselves have a relatively low electrical conductivity and, hence, the resulting electrode remains to be of relatively low conductivity even when the percolation threshold is reached. A relatively high proportion of CBs (far beyond the percolation threshold) must be incorporated in the cathode to make the resulting composite electrode reasonably conducting.
Clearly, an urgent need exists for a more effective electrically conductive additive material. Preferably, this electrically conductive additive is also of high thermal conductivity. Such a thermally conductive additive would be capable of dissipating the heat generated from the electrochemical operation of the Li-ion battery, thereby increasing the reliability of the battery and decreasing the likelihood that the battery will suffer from thermal runaway and rupture. With a high electrical conductivity, there would be no need to add a high proportion of conductive additives.
There have been several attempts to use other carbon nano-materials than carbon black (CB) or acetylene black (AB) as a conductive additive for the cathode of a lithium battery. These include carbon nano-tubes (CNTs), vapor-grown carbon nano-fibers (VG-CNFs), and simple carbon coating on the surface of cathode active material particles. The result has not been satisfactory and hence, as of today, carbon black and artificial graphite particles are practically the only two types of cathode conductive additives widely used in lithium ion battery industry. The reasons are beyond just the obvious high costs of both CNTs and VG-CNFs. The difficulty in disentangling CNTs and VG-CNFs and uniformly dispersing them in a liquid or solid medium has been an impediment to the more widespread utilization of these expensive materials as a conductive additive. Additionally, the production of both CNTs and VG-CNFs normally require the use of a significant amount of transition metal nano particles as a catalyst. It is difficult to remove and impossible to totally remove these transition metal particles, which can have adverse effect on the cycling stability of a lithium metal.
As for the less expensive carbon coating, being considered for use in lithium iron phosphate, the conductivity of the carbon coating (typically obtained by converting a precursor such as sugar or resin via pyrolyzation) is relatively low. It would take a graphitization treatment to render the carbon coating more conductive, but this treatment requires a temperature higher than 2,000° C., which would degrade the underlying cathode active material (e.g., LiFePO4).
As an alternative approach, Ding, et al investigated the electrochemical behavior of LiFePO4/graphene composites [Y. Ding, et al. “Preparation of nano-structured LiFePO4/graphene composites by co-precipitation method,” Electrochemistry Communications 12 (2010) 10-13]. The co-precipitation method leads to the formation of LiFePO4 nano-particles coated on both primary surfaces of graphene nano-sheets. The cathode is then prepared by stacking these LiFePO4-coated graphene sheets together. This approach has several major drawbacks:                (1) With the two primary surfaces of a graphene sheet attached with LiFePO4 nano-particles, the resulting electrode entails many insulator-to-insulator contacts between two adjoining coated sheets in a stack.        (2) Only less than 30% of the graphene surface area is covered by LiFePO4 particles on either side. This is a relatively low proportion of the cathode active material.        (3) The LiFePO4 particles are easily detached from graphene sheets during handling and electrode production.        (4) We have found that the nano particle-attached graphene sheets as prepared by the co-precipitation method are not amenable to fabrication of cathodes with current electrode coating equipment. In particular, these particle-attached graphene sheets could not be compacted into a dense state with a high mass per unit electrode volume. In other words, the cathode tap density is relatively low. This is a very serious issue since all of the commonly used cathode active materials, including LiFePO4, already have a very low specific capacity (mAh/g), and not being able to pack a large mass of a cathode active material into a given electrode volume would mean an excessively low overall capacity at the cathode side. (It may be noted that the typical specific capacity (140-170 mAh/g) of a cathode active material is already much lower than that (330-360 mAh/g) of an anode active material. Such an imbalance has been a major issue in the design and fabrication of lithium ion batteries).        
Thus, it is an object of the present invention to provide a thermally and electrically conductive cathode active material that can be easily incorporated in a cathode electrode of a lithium battery.
A specific object of the present invention is to provide a cathode active material-coated discrete graphene sheets (as primary particles) that readily aggregate into secondary particles (herein also referred to as particulates) that are more amenable to mass production of cathodes using current production equipment.
A particularly desirable specific object of the present invention is to provide cathode active material-coated graphene sheets that are more conducive to the formation of a 3-D network of electron-conducting paths, imparting exceptional conductivity to the cathode and enabling the cathode to become high-rate capable.
The cathode active material-coated graphene sheets disclosed herein, typically 0.3 μm-10 μm long or wide, have a graphene sheet thickness in the range of 0.5 nm to 10 nm and cathode active material coating thickness in the range of 2 nm to 100 nm (more typically 5-50 nm). The cathode active material loading (percentage) is typically >80%, more typically >90%, and most typically 95-99%. This active material proportion is readily adjustable.
Another object of the present invention is to provide a process to produce cathode active material-coated graphene sheets.
A further object of the present invention is to provide a cathode electrode that has a high cathode active material proportion and a rechargeable battery that contains such a cathode electrode.